The customary pumping arrangement employed in sewage lift stations and the like is comprised of at least one self-priming centrifugal pump, a suction inlet pipe connected to the intake side of the pump, a discharge main connected to the discharge side of the pump, and a one-way check valve in the discharge main which prevents liquid from flowing back to the pump when it is shut down.
When the pump is started up, the air in the pumping chamber and any air in the suction inlet pipe must be evacuated in order to achieve a full prime. The air which is evacuated cannot be forced past the check valve in the discharge main because self-priming pumps have limited air compression capability. For example, a typical four inch self-priming centrifugal pump, when in priming mode, may develop a maximum pressure of 7 feet of head at its rated speed. If such a pump is installed in a system with a discharge check valve that requires a pressure in excess of 7 feet to open, the pump will not develop sufficient pressure to open the check valve and initiate flow. It is therefore necessary to provide an air release valve between the check valve and the pump for venting air from the system. In order to have an efficiently operating system, the air release valve should automatically close when the pump is operating at rated capacity and head.
Many types of air release valves have been proposed. Many of these valves can become unstable under certain operating conditions, for example, low pressure conditions, and are prone to hydraulic chattering when pressure conditions are varying. Another drawback to known air release valves is that they can be easily plugged or fouled by stones, sticks, stringy material, an other solids commonly found in raw sewage and the like.